2. 3 Patients
We included children aged 7–15 years who were prescribed antiviral drugs for influenza in an outpatient setting during the 2018/2019 influenza season (December 1, 2018 to March 31, 2019). During this period, baloxavir was approved for children whose body weight was ≥10kg without age restriction, and was available only as a tablet; we nonetheless did not include young children in this study, assuming that they would have had difficulty in swallowing a pill. The patients were included into the cohort by the date of prescription (the index date), and were followed 1–9 days after the index date; this window was selected as comparable to those of baloxavir clinical trials [6,14]. If a patient received repeated prescriptions for antivirals after this period ended, we regarded as the event as a separate infection and entered the patient into the cohort again.
Underlying comorbidities conferring a high risk of influenza complications were examined for each patient by ICD-10 code. These included asthma or chronic lung diseases, metabolic and endocrine disorders, neurological and neurodevelopmental disorders, heart diseases, and blood disorders (Supplemental Materials). These comorbidities were prespecified, harmonizing the eligibility criteria of CAPSTONE-2 trial (a phase 3 baloxavir trial conducted exclusively in patients at risk of severe influenza) and selected literature relevant to complications of influenza infection [14-16]. To correctly specify the presence of these comorbidities, patients with JMDC data history < 12 months before the index date were excluded from the study.